The Bojon Gourmet

Crust
0 Prepare and parbake the crust as directed. 
10 Separate one of the eggs, beat the white well, and brush the parbaked crust with a thin layer of egg white. Return the crust to the oven for 3-5 minutes to set the white. This will create a barrier between crust and custard and keep the crust crisp. Reserve the remaining egg white and yolk to use in the custard.
Veggies
1 To prepare the vegetables, place a wide skillet over medium-low heat. Add the oil, leeks, and ¼ teaspoon salt and cook, stirring frequently, until very tender but not brown, 10-15 minutes, reducing the heat as needed. Remove the leeks to a bowl.
2 Add the spinach to the hot skillet in batches, stirring frequently, until all of the spinach is softly wilted and dark green, 2-3 minutes. Transfer the spinach to a strainer and press it with the back of a silicone spatula or large spoon to remove as much of the liquid as you can. You should have about 1 cup of cooked spinach.
Custard Filling
3 To make the custard, place the flour, ¾ teaspoon salt, and pepper in a large bowl. Add one of the eggs, whisking well to incorporate the flour, then whisk in the remaining eggs (including what’s left of the egg used for brushing the crust) one at a time. Gradually whisk in the milk and cream. Whisk gently so as to not incorporate too much air.
Assemble & Bake
4 Turn your oven to 350ºF and position a rack in the lower third of the oven. Have the parbaked crust on a baking sheet. 
14 Spread half of the cheese in the bottom of the crust, then all of the leeks, all of the spinach, and the remaining cheese. 
15 Give the custard another stir to make sure the flour is incorporated, then pour it into the crust; it will be quite full. (If you have a lot of custard left over, remove some of the filling ingredients and bake the extra custard and filling in greased muffin cups for a little treat.)
5 Carefully transfer the quiche to the 350º oven and bake until the top is golden and puffed, and a knife inserted near the center reveals no liquid, 45-60 minutes. Rotate the quiche once or twice for even baking. Take care not to overbake or the custard will turn grainy.
Serve
6 Let the quiche cool to warm or room temperature to make it easier to slice, at least 20 minutes and up to several hours. Use a sharp knife to cut the quiche into wedges. I found a small serrated knife to work especially well. Or for cleaner cuts, chill the quiche first, then reheat individual slices. Serve warm or at room temperature.
7 The baked quiche will keep at cool room temperature for up to 3 hours. Extra quiche can be wrapped and refrigerated for up to 4 days. To reheat, place in a 350º oven or toaster oven until heated through, 10-15 minutes.
8 To freeze quiche, place cooled slices in an airtight freezer-safe container and freeze for up to 3 months. To reheat, place slices in a 350º oven or toaster oven until heated through, 15-20 minutes or as needed.